1. Field of the Invention
The present invention relates to improvements in plungers used in a gas/fluid lift system in wells producing both fluids and gases, such as petroleum and natural gas, under variable pressure to facilitate the lifting of fluids from a subterranean reservoir to the surface through a well conduit or tubulars. Plungers of this type are designed to minimize the downward flow of fluids as well as the upward flow of gases beneath the plunger as the plunger travels upwardly to the surface. Tubulars include, but are not limited to, a variety of tubes and tubular members, such as cement casings, conduits, tubing and tubing strings which are placed in the well conduit, and may also be referred to as the production string. More specifically, the gas plunger invention concerns improvements in the internal and external sealing of the apparatus. The external sealing means or apparatus is typically comprised of a plurality of segments, which collectively forms a jacket assembly that sealingly and slidingly engages the well tubulars. A turbulent inner seal is accomplished by sealing means such as circumferential grooves on the inner core and/or fingers which project inwardly from the segments toward the inner core which may or may not be grooved. Alternatively, the inner surface of the segments may have furrows and there may be raised bands on the core which also effects a turbulent inner seal. The circumferential grooves and/or fingers, or the bands and/or furrows, provide a tortuous path of flow that deflects escaping gas streams and/or fluids, promotes turbulence in the manner of a labyrinth seal, and has gas sealing capabilities.
Another further and alternative improvement concerns a simplified sucker rod and valve-like assembly used to regulate and restrict the flow of fluids and gases through the internal passage of the plunger which allows such plungers to descend to the well bottom more rapidly than plungers without internal passages so that flow occurs only during the downward cycle or descent of the gas plunger.
2. Description of the Prior Art
Differential gas pressure operated pistons, also known as plungers, have been used in producing subterranean wells where the natural well pressure is insufficient to produce a free flow of gas, and especially fluids, to the well surface. A plunger lift system typically includes tubulars placed inside the well conduit, which extend from the reservoir(s) of the well to the surface. The tubulars have a well valve and lubricator at the top and a tubing stop and often a bumper spring or other type of spring assembly at the bottom. The cylindrical plunger typically travels between the bottom well stop and the top of the tubulars. The well is shut in for a selected time period which allows pressures to build up, then the well is opened for a selected period of time. When the well valve is opened, the plunger is able to move up the tubulars, pushing a liquid slug to the well surface. When the well valve is later closed, the plunger, aided by gravity, falls downwardly to the bottom of the tubulars. Typically, the open and closed times for the well valve are managed by a programmable electronic controller.
When the plunger is functioning properly, fluids accumulate and stay above the plunger and pressurized gases and/or fluids below the plunger are blocked from flowing up, around, and through the plunger. As a result, the plunger and accumulated fluids are pushed upwardly. The prior art devices use a variety of external, and sometimes internal, sealing elements which allow the plungers to block the upward flow of gases and slidingly and sealably engage the tubulars, which accomplishes the lifting of fluids to the surface depending upon the variable well pressures. Examples of prior art gas operated plungers include those disclosed in U.S. Pat. Nos. 5,427,504 and 6,045,335 (hereinafter the ""504 and ""335 patents). The prior art plunger of the ""504 patent features mechanical sealing which is accomplished by segments that are biased outwardly against the tubulars by springs. The build up of internal pressure is accomplished by a flexible, elastomeric seal placed beneath the segments. The outer sealing assembly is comprised of a plurality of segments or pads. However because such resilient compounds like rubber do not last for extended periods of time in the harsh well environment, problems with inner sealing develop and the plunger must be taken out of service for time-consuming seal replacements. Further, if the inner spring member which assists in biasing of the segments becomes detached or lost, sealing problems could result.
In contrast, the prior art plunger of the ""335 patent has upper and lower sets of segments whose sides are juxtaposed with respect to each other and collectively work together. The segments are biased outwardly against the tubulars by springs and the build up of internal pressure. The sealing element therein consists of a rigid inner ring member surrounding the intermediate portion of the piston body, which is positioned between the piston body and between the inner surfaces of each set of cylindrical segments, which cooperate to slidingly engage the rigid ring member and create an inner seal. However, the segments of this design can be prone to leakage.
Other prior art plungers which have externally grooved surfaces and which lack outer sealing elements or segments are, for example, disclosed in U.S. Pat. Nos. 4,410,300 and 6,200,103. These external grooves deflect the escaping gas streams and promote turbulence in the manner of a labyrinth seal and have gas sealing capability. However, the grooves are prone to structural failure due to external wear and erosion due to contact with the tubulars, and these plungers can also become jammed within the tubulars because these types of plungers do not have the capability of contracting radially inward, as do the plungers with cooperating mechanical sealing segments. The improved plunger design incorporates the concept of a labyrinth seal in its internal sealing elements.
Other examples of prior art gas operated plungers include those with internal bores or passages to speed the descent of the plungers. These plungers have a variety of valve closure members which seal the internal bore, and the prior art valve closure members are often spring loaded and work in conjunction with long rods which typically extend downwardly through the bore to unseat the valve closure member, as disclosed in the ""504 and ""335 patents. The design of the piston disclosed in the U.S. Pat. No. 6,045,335 includes a complicated valve mechanism which requires a unit to capture the piston at the surface and requires a long rod which moves downwardly through the plunger bore to disengage and unseat the valve closure member, and to open the internal valve. However, this rod used to reopen the valve assembly is prone to damage and bending if the rod and plunger bore become even partially unaligned, requiring expensive and time-consuming repair or replacement. Additionally, this type of plunger also requires expensive and customized installation of equipment at the well surface such as spring loaded stops to accomplish disengagement of the valve closure member. In contrast, the plunger of the ""504 patent has a bypass valve with a ball-shaped closure member and a spring loaded rod activator, or shock spring, which pushes the ball up into the valve seat to seal off the flow path. The spring loaded rod activator opens the valve after the plunger reaches the lubricator at the top of the well and the pressures above and below the plunger are equalized.
In contrast, the improved stopper assembly which is housed in a chamber is typically located in a modified end cap and seals off the inner passage in a simplified manner. The stopper stem and stopper head is pushed up into the chamber when the plunger bottom contacts the well stop means, and the stopper is held up against the opening of the inner passage by the fluid and/or gas pressure below the plunger. This simplified and improved design dispenses with the need for complicated moving parts which to actuate the closure means, and eliminates the need for expensive equipment at the well head which is used to unseat the closure means.
The improved plunger inventions seek to dispense with the problems of the prior art such as erosion, leakage, erratic or unsafe operation, malfunctions, and costly replacements or repairs. Many other objects and advantages of the inventions, besides substantially trouble free operation, will be apparent from reading the description which follows in conjunction with the accompanying drawings.
The present invention provides a plunger for use in a gas/fluid lift system in tubulars in wells producing both fluids and gases under variable pressure. The plunger assists with the build up of pressure between the subterranean reservoir and the surface by having an inner seal and an external sliding and variable holding seal with adjacent well tubulars. The inner and external seals restrict the upward flow of the fluids and/or gases. This causes an increase in the well pressure below the plunger and facilitates the upward lifting of the plunger and fluids from the reservoir to the surface when pressure is reduced above the plunger, such as at the well head, The improved plunger comprises a body which is slidingly engageable and which gravitates within the tubulars. The plunger body has an external sealing means such as a plurality of segments which are mounted around a core, also known as a mandrel, and which collectively form a jacket. The segments, collectively the jacket assembly, are slidingly and sealingly engageable with the insides of the well tubulars, based upon the pressure effected between the inner surface, or inside, of the jacket and the core. The jacket has the largest diameter of the plunger when the segments are in an expanded radial position. The segments have a convex outer surface and typically have a concave inner surface. However, the core of the plunger could be square, triangular, or of another geometric shape, in which case the inner surfaces of the segments could be flat, or of any other corresponding geometric shape.
In a preferred embodiment of the plunger, there is also an inner sealing means such as at least one rigid finger which projects radially inward from the underside of each segment toward the core, with the fingers of the adjacent segments collectively cooperating to encircle the core. Preferably, there are a plurality of fingers on the undersides of each segment. The fingers are normally separated from the core especially when the segments, collectively the jacket, are pushed radially outward. This creates a path of flow for gases and/or liquids and the fingers collectively create a tortuous path of flow between the core and the segment undersides and effect a turbulent inner seal. When the segments making up the jacket are pushed to their most radially inward position, the fingers touch the core and cause a complete inner seal. In another embodiment of the plunger, the core has at least one circumferential groove on its surface, and more preferably a plurality of grooves. This also creates a tortuous path of flow between the core and the jacket underside and effects an inner seal. In another embodiment, the plunger has both grooves and fingers, and the fingers are correspondingly located to fit into the grooved portions of the core. This design creates an even more tortuous path of flow for fluids and gases which effects an inner seal and creates an increased surface area between the segments and core. The increased surface area also has the effect of increasing the internal plunger pressure, i.e., the pressure between the core and the jacket assembly and energizes the segments, pushing the segments radially outward toward the well tubulars. This preferred design also prevents detachment and/or loss of the segments if the retainer rings, explained below, fail because the segments will be held in place by the finger-groove interface and by the outer well tubulars. This design provides for increased functionality and seeks to minimize expensive and time consuming fishing operations to retrieve dislocated parts.
An alternate embodiment also has at least one biasing means, which is typically a spring, between the underside of each segment and the core to outwardly bias each segment and to achieve inward and outward radial rebounding of the segments from the inner core. The preferred embodiment also has recessed spaces, or blind holes, in the core or core grooves and/or the fingers which hold the biasing means in place between the core and segments and prevent displacement and loss of the biasing means. The preferred embodiment typically also has retaining means such as retaining rings which limit the outward radial movement of the segments/jacket assembly. In plungers with both fingers and grooves, at least one of the outside edges of the grooves will be angularly reduced to allow installation of segments with projecting fingers into the grooves of the core and allows the end of the segments to be installed underneath the retaining rings.
In yet another embodiment of the invention, the plunger has an internal passage which extends partway through the body, or through the entire axis of the plunger, to facilitate more rapid descent of the plunger to the bottom of the well or the well stop means. These plungers also have a top end and a bottom end with at least one opening at or near the top and the bottom end and may have a plurality of radial ports which connect to the bore to increase the flow rate and to facilitate even more rapid descent of the plunger. The preferred embodiment has a plurality of radial ports near the top end and bottom end. These plungers further have a chamber in a modified end cap near the bottom end which houses a closure means such as a plunger stopper. The chamber connects to the internal passage at the roof and connects to the stem bore in the floor of the chamber. The plunger stopper has a top end which has a shape similar to that of the roof, or upper chamber area, and has a stem attached to the bottom end which extends downward through and protrudes outwardly from a bore opening in the bottom end. When the stem engages the bottom well stop means upon descent, the closure means such as a stopper, is pushed upwardly against the roof of the chamber, thereby sealing off the inner passage and restricting the upward flow of fluids and/or gases in order to build up pressure below the plunger. The improved design of this closure means, or stopper, operates without springs or catches, yet still holds the stopper against the roof of the chamber. It also does not use long sucker rod, which are prone to bending, to unseat the closure means. Instead, the pressure build-up below the plunger keeps the plunger stopper engaged against the roof of the chamber. The simplified bore sealing means also reduces the amount of time needed for costly and time-consuming repairs and replacements and dispenses with the need for expensive and customized devices at the surface that unseat the prior art closure valves.
The preferred embodiments of this invention may also have the previously described advantages of the rigid fingers, the grooved core, the spring recesses, and the reduced edge of the core groove. In another preferred embodiment of the invention, the top end of the closure means, such as the plunger stopper, also has a stem which is pushed upward into the inner passage above the chamber roof to further seal off the inner passage.